My 90 3vze auto runner has some ugly tranny fluid. however it still shifts fine and the fluid isn't burnt. thought about changing the fluid and filter, but have heard that supposedly in older trannys that this could be a bad idea. something about knocking the gunk that was "holding" the tranny together loose and frying it

Still not used to an auto (everything else ive owned for the past 6-7 years has been a stick)

 

Just do a drain and fill, don't flush it
I am unclear as to how much a drain and fill takes but I had a 95 4Runner and I did a drain and fill and the tranny was much more responsive
At most you are looking at a few quarts

 

If the fluid is worn out, change it!

If you're worried about your seals, try Castrol Import Multi-Vehicle ATF or Chevron Multi-Vehicle ATF - both of those are Dexron II/III compatible fluids with a bit higher viscosity. Or you could use Red Line D4 ATF - the synthetic polyol ester base stock that they use will condition the seals. The synthetic fluid is more shear stable and will can give you a longer drain interval.

http://www.redlineoil.com/product.aspx?pid=51&pcid=9
http://www.castrol.com/liveassets/bp...s_ImportMV.pdf
https://www.cbest.chevron.com/genera...DS27291691.PDF

 

he should definitely change it but I would advise against a complete flush and filter change, a drain and fill or two would probably be best

 

definately wasn't going to flush! heard alot of bad things about that! would much rather just do a 5 speed swap, but can't take the truck off the road as it's my daily (mustang is far too much of a cop magnet with flowmasters and a cam!)

So what i was thinking was filter change and drain. replacing fluid with a good synthetic (amsoil, red line or royal purple) this should be ok? I have the mechanical knowledge, just want any auto trans specific suggestions

 

Check the bobistheoilguy forum for differences between. They love Amsoil ATF, seem to hate Royal Purple, and they have less to say about Red Line but what they say is very positive. Reason I mentioned the Red Line is because of your expressed concern for seals. The Red Line is a safer bet in that regard, but there's a good chance Amsoil would work without problems and its an excellent fluid. If you're going synthetic, I'd choose the Red Line or Amsoil.

 

If you are only doing a drain in fill then going to synthetic seems worthless to me because you are going to end up with a mix that is mostly conventional. If you are against flushing (and I don't understand the logic) and insist on using synthetic do a drain and flush at least twice, probably three times to get the mix to be mostly synthetic.
I don't really understand the hate against the flush, it's not like an engine flush no harsh solvents are used. They simply run all of the old ATF out and replace it with new. I have had this done twice on my 99 GMC 1500 with no ill effects and everytime it improves shifting.
Just my $.02